Coating powders having enhanced electrostatic chargeability

ABSTRACT

Inert nitrogen-containing compounds, such as melamine, urea, dicyandiamide, and benzoguanamine and derivatives of such compounds are added to thermosetting resin powder coatings to enhance the electrostatic charge of the powders. Such enhancement improves coating processes for non-conductive and conductive substrates.

[0001] The present invention relates to thermosetting resin coatingpowder compositions having enhanced electrostatic chargeability. Thecoatings are applied using conventional electrostatic equipment and thenfused and cured. The coating powder compositions of the inventioninclude inert nitrogen-containing compounds that function aselectrostatic charge enhancing agents. By inert, it is meant that thesecompounds are not chemically reactive with other compounds of thecoating powder under the conditions the compounds are normally exposed,i.e., during powder manufacturing and coating. Melamine, urea,dicyandiamide, and benzoguanamine and derivatives thereof are inertnitrogen-containing compounds that enhance chargeability. The use ofthese coating powders eliminates or minimizes the need for the surfacepreparation of non-conductive substrates prior to electrostaticapplication and serves to improve coating thickness, uniformity, andquality.

BACKGROUND OF THE INVENTION

[0002] Coating powders are extremely desirable for coating substratesbecause such powders are virtually free of the fugitive organic solventsthat are conventionally used in liquid paint systems. Powder coatings,thus, emit few, if any, volatile materials to the environment when heator radiation cured. Problems of air pollution and health dangers toworkers employed in painting operations are thereby reduced.

[0003] Solventless fusion coating processes have been developed to applycoating powders on substrates in which dry, finely divided, free flowingheat fusible powders are deposited on the substrates and then fused andcured with heating to form continuous, protective or decorative films.Examples of such coating processes include electrostatic spray andfluidized bed techniques; with the electrostatic spray technique beingmore frequently used in industry.

[0004] Although powder coatings have many benefits, it is difficult tocoat electrically non-conducting substrates such as plastic, wood, andwood composites. The two main challenges are the need for low curingtemperatures and the need for enhancement of the electrostaticattraction of coating powder to non-conductive substrates. The pastseveral years, new technologies have been introduced into the market forproviding low temperature cure powder coatings. U.S. Pat. Nos. 5,714,206and 6,077,610 involve two-component coating powders that can be cured atthe low temperatures required for wood and a variety of other metallicand non-metallic substrates. This result is achieved by mixing resin andcuring agent components following extrusion in the dry form. Theprocedure eliminates heating, and thus reaction, of the two componentstogether until they are deposited on the substrate. U.S. Pat. No.5,721,052 discloses epoxy resins cured with imidazole adducts to obtainlow temperature cure powder coatings, especially black texturedcoatings. U.S. Pat. No. 5,824,373 discloses UV curable powder coating tofurther reduce cure temperatures through the use of UV radiation curing.All of the above patents address the low cure temperature aspect of thepowder coatings for non-conductive substrates and thus address theproblem of substrate degradation at normally used higher powder coatingcure temperatures. A further challenge remains in the enhancement of theattraction of coating powders to non-conductive substrates prior tocuring.

[0005] Powder attraction techniques, such as those discussed below, areknown in the art.

[0006] U.S. Pat. No. 5,731,043 discloses producing coating powders ofcontrolled particle size distribution to improve tribo-chargingcharacteristics for wood and metal substrates. Several other techniqueshave been developed to impart sufficient electrical conductivity to thesubstrate so that the substrate can be electrostatically powder coated.A conductive material, such as graphite, can be added to the substrateto improve substrate conductivity. However, this technique has thedisadvantage that modification of the character of the substrate isrequired.

[0007] European Patent Publication No. 0933140A1 discloses a methodwhere the substrate can be preheated to cause the powder particles topartially cure and adhere when the particles initially contact theheated surface. Preheating wood substrates also assists in drivingmoisture to the surface and thereby assisting the electrostaticapplication. Due to the varying nature of the wood substrates,controlling the moisture content is very difficult. Should the moisturecontent be too low, even preheating the part will not sufficientlyimprove electrostatic application. Excessive preheating to improveconductivity can also be detrimental to the substrate. Moreover,preheating parts that have sharp edges permits moisture to escape morerapidly from such edges than from thicker areas, thereby renderingpowder application very difficult.

[0008] A typical solution to the electrostatic application problem is toapply a conductive primer to the substrate prior to powder application.This approach is illustrated in U.S. Pat. No. 5,344,672 where anelectrically conductive primer, typically containing metallic orgraphite particles, is coated onto the surface of the substrate.Although such approach is operable, it interposes an electricallyconductive coating between the substrate and the cured powder coatingand thus requires an additional process step that is not required by thepresent invention. Electrically conductive coatings can interfere withsome intended uses of the finished part, which otherwise would notexhibit electrical conductivity. The present invention does not requiresuch conductive primer.

[0009] Still another approach involves the application of an antistaticmaterial to the substrate prior to coating powder application. PatentPublication No. WO98/58748 and U.S. Pat. No. 4,686,108 disclose that theuse of conductive polymeric coatings having a charge density of greaterthan 2, after being applied to nonconductive substrate, permitssubsequent overcoating of the substrate by electrostatic spraying. Theantistatic coating, typically on the order of a few micrometers thick orless, provides sufficient electrical conductivity to the surface topermit electrostatic powder coating. The surface conductivity of theantistatic-coating substrate is about 10¹² ohms per square or more, andmay be adjusted by heat treatments. Such high resistivity does notresult in unacceptable electromagnetic wave attenuation for most end useapplications. A limitation of this approach is that multiple steps areinvolved in finishing operations. The present invention does not requiresuch antistatic materials.

[0010] Antistatic agents most commonly are amines, such as tertiaryamines, or ammonium compounds, such as quaternary ammonium compounds.These agents cannot be incorporated into coating powder formulationsbecause such agents are commonly used at very low levels (less than 0.5part per 100 parts of resin) as catalysts in many coating powderformulations. This approach is limited with respect to the amount ofmaterial that can be incorporated in the formulation because ammoniumcompounds also function as cure catalysts for most coating powderformulations.

[0011] Finally, U.S. Pat. No. 6,113,980 discloses improvingchargeability of powder coatings using electron donor compounds forcoating metal substrates by tribocharging the coating powders. Certainhindered amines are disclosed as such compounds. The implementation ofthis approach has been hindered by the lack of commercial availabilityof the charge control agents as well as the cost of these materials. Thechemical structure of these hindered amine compounds can impartperformance characteristics to the coating powders that may not bedesirable, at least in some formulations. The patent requires that theelectron donor compounds comprise at least two different compoundscontaining a residue of the following formula:

[0012] On the other hand, the present invention utilizes electrostaticcharge enhancement compounds for powder coating non-conductivesubstrates that are quite structurally distinct from that of said U.S.Pat. No. 6,113,980. Moreover, the present invention requires only asingle ingredient and is thus less cumbersome.

[0013] There remains a need for further improved technique to enhancethe electrostatic powder coating of electrically non-conducting andelectrically conducting substrates. Such technique would find widespreadapplication in the coating of, for example, wood, wood compositematerials, ceramics, glass, plastics, and the like as well as metal. Itis believed that such enhancement not only fulfills a long-standing needin the art but also provides additional advantages.

SUMMARY OF THE INVENTION

[0014] The coating powders of the invention include as a component aninert nitrogen-containing compound such as melamine, urea,benzoguanamine, dicyandiamine, and derivatives of such compounds. Theterm “inert” means that the compounds do not react with other componentsof the coating powder during manufacturing steps of the powder, such asextrusion, or during coating temperatures when the powder is applied toa substrate. Thus, the essentially unreacted compound is free to performits function as an electrostatic charge enhancement agent during thecoating process. The balance of the powder is a thermosetting resinpolymer that may contain other ingredients that are typically includedin coating powders.

[0015] In one advantageous aspect, the process of the invention involvescoating non-conductive substrates by providing the above describedcoating powder, electrostatically charging said powder, applying thecharged powder to a non-conductive substrate, and then curing theapplied powder by use of heat or radiation. Curing is conducted attemperatures below about 300° F. for heat sensitive substrates such aswood and plastic to avoid damage to such substrates. Curing temperaturesgreater than about 300° may be used for other non-conductive substratescapable of withstanding higher curing temperatures such as glass,ceramics, etc. Because of the enhancement of chargeability of thecoating powders, conductive substrates, such as metals, may also becoated. The prsent invention is especially useful where thin or complexcross-sections are being coated because coating uniformity is enhancedby use of the coating powders of the present invention. The coatedarticle of the present invention results from the above-describedprocess.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] The present invention generally involves, in one aspect, lowtemperature curing, thermosetting coating powders containing inertnitrogen-containing compounds such as melamine and others, whichfunction as electrostatic charge enhancement agents. Such coatingpowders are applied to non-conductive, heat sensitive substrates such aswood and plastic, and then cured at temperatures below about 300° F. Theinertness of the compounds (lack of reaction during extrusion andcoating powder application to the substrate permits the compounds to beavailable to function as electrostatic charge enhancement agents. Suchcoating powders can be applied, particularly by electrostatic spraycoating, to non-conductive substrates without need for primers orexcessive heating of the substrate. Such coating powder formulationsflow freely during application and cure at low temperatures withoutdamage to the substrate.

[0017] Enhanced electrostatic charging characteristics are developed byincorporating nitrogen-containing inert materials, such as melamine,urea, dicyandiamide and benzoguanamine, and derivatives thereof, intothe thermosetting coating powders. The inert characteristic of thesematerials is derived from a high melting point and/or chemicalreactivity. If they have low melting point and/or chemical reactivity,other compounds will react with the resin system and cause prematurereactions in the extruder and unwanted changes to the coating appearanceas well as decrease storage stability of the powder coating. Thesecompounds need to be used at levels much higher than the catalytic usagelevels. The incorporation of charge-improving agents into the coatingpowder composition eliminates multiple steps otherwise required toproduce a high quality finished part in the coating process. Theelectrostatic charge enhancing agents of the invention are readilyavailable at very low cost. Another important result of the invention isthat appearance and performance of the coating is not altered, even atvery high loading of the electrostatic charge enhancing agents. Enhancedelectrostatic charge also leads to thicker coatings, improved coatinguniformity, as well as to improved coating at part edges and at thinnercross-sectional areas of the part.

[0018] The above-mentioned electrostatic charge enhancement agents maybe incorporated into a variety of thermosetting resin polymers includingepoxies, saturated polyesters, unsaturated polyesters containing asurface curing initiator, polyester-epoxy hybrids, acrylics, andadmixtures thereof. Curing agents and/or catalysts for such resins areselected to obtain a curing temperature of about 300° F. or less forheat sensitive substrates to avoid damaging such substrates duringcuring. Curing temperatures in excess of 300° F. may be used forsubstrates such as glass, ceramics, ceramic tiles, graphite, carbon,metal, and the like, which would not be damaged at curing temperaturesover 300° F.

[0019] Low-temperature curing epoxy resin systems such as set forth inU.S. Pat. Nos. 5,714,206 and 5,721,052, are suitable for use in thisinvention. Both systems are curable at temperatures of 300° F. or below.

[0020] Epoxy resins included in U.S. Pat. No. 5,714,206 are exemplifiedby, but not limited to, those produced by the reaction ofepichlorohydrin and a bisphenol, e.g., bisphenol A. Preferred epoxyresins include those sold under the trademarks ARALDITE GT-7072, 7004,3032, 6062, and 7220, and EPON 1007F, 1009F, and 1004, all of which are4,4′-isopropylidenediphenol-epichlorohydrin resins.

[0021] The epoxy resin is self-curing, i.e., it reacts viahomopolymerization during curing of the powder coating. Generally, acatalyst is required to cause the reaction to progress at a commerciallyacceptable rate. A suitable catalyst is an epoxy adduct of an imidazolehaving the general formula:

[0022] Wherein R¹, R², R³, and R⁴ are independently hydrogen or anysubstituent, which is not reactive with the epoxy resin. Examples ofsuitable imidazoles with a bisphenol A epoxy resin are availablecommercially from Shell Chemical Company under its trademark EPON, e.g.,EPON P-101, and also from Ciba-Geigy corporation under its designationHT 3261. The term imidazole includes both substituted and unsubstitutedimidazoles. The adducted imidazole acts as a catalyst, moving from oneepoxy group to another as it facilitates epoxy ring opening and curereactions. Imidazoles, in themselves, tend to be insoluble in epoxyresins. Thus, the purpose for adducting them to an epoxy resin is tomake them compatible with the epoxy system. As a catalyst, the imidazoleadduct is used at a level of from about 0.1 to about 8 parts per hundredparts of the extruded resin. For enhanced color stability, the 2-phenylimidazole may be used as the catalyst for curing the epoxy resin with orwithout the low temperature curing agent. The 2-pheyl imidazole, whichis available from the SW K Chemical Co., may be used as such ataccordingly lower levels.

[0023] Imidazoles, as adducts or non-adducts, may also be used at higherlevels as a separately added curing agent to the extruded mixture of theresin and catalyst. When this is done, the amount of imidazole adduct iscontrolled so that the total amount is no more than about 12 phr.

[0024] Otherwise, the low temperature curing agent may be selected fromamong the many that are commercially available but an epoxy adduct of analiphatic polyamine having a primary amino group is preferable. Asuitable curing agent of that type is available from Ciba-Geigy as itsHT 835 hardener. A similar product is sold under the trademark ANCAMINE2337 XS by Air Products & Chemicals. An epoxy adduct of an aliphaticpolyamine having a secondary amino group available under the trademarkANCAMINE 2014 AS is preferred for white and light colored coatings. Theamount of low temperature curing agent that may be added separately ascomponent (B) to the pulverized extrudate of resin and catalyst is fromabout 2 to about 40 phr and the preferred amount is from about 30 toabout 35 phr. The ratio of the low temperature curing agent to thecatalyst in the extrudate is from about 1:3 to about 400:1 butpreferably from about 2:1 to about 15:1.

[0025] The epoxy resin system shown in U.S. Pat. No. 5,721,052constitutes another example of a system that can typically be used inthis invention. Such epoxy resins are solid resins or blends of solidand small amounts of liquid resins up to about 10 wt. % which resins arethe reaction products of a diol and a halohydrin. Suitable epoxy resinsare exemplified by, but not limited to, the reaction products ofbisphenol A and epichlorohydrin. Generally, the bisphenol A type epoxiesused herein are of the type 1 to type 9 form, with the low viscositytype 3 or less epoxy resins being most preferred. The useful bisphenol Atype epoxy resins have an epoxy equivalent weight ranging between about400 and 2.250, preferably an epoxy equivalent weight of between about550 and 1.100, with an epoxy equivalent weight of between about 600 and750 being most preferred. Preferred epoxy resins include those sol underthe tradename Araldite GT 7013 (type 3) and Araldite GT 7072 (type 2) byCiba-Geigy corporation, which are both4,4′-isopropylidene-diphenol-epichlorohydrin type epoxy resins.

[0026] The thermosetting powder coating compositions described above forboth epoxy systems contain as another component, a catalytic curingagent. Such catalytic curing agent serves the dual function of curingagent and cure accelerator. No other curing agents need to be present inthe powder coating compositions of the present invention. The advantageof this catalytic curing agent component is that it alone allows thecoating powders to cure at lower cure temperatures or at ultra rapidcure rates, thereby permitting such compositions to be coated on heatsensitive materials, especially wood and plastic substrates, withoutdeteriorating the physical and/or chemical properties of the substrate.

[0027] An imidazole and epoxy resin adduct is sold under the tradenameEpon Curing Agent P-101 by Shell Chemical Company can be used in thepresent invention. Another preferred imidazole adduct of a bisphenol Atype epoxy resin is sold under the t5radename HT 3261 by Ciba-GeigyCorporation. The imidazole or substituted imidazole residue, e.g., a2-methylimidazole residue, typically comprises between about 5 and 50wt. % of the imidazole/epoxy resin adduct composition.

[0028] It is believed that the epoxy component of the imidazole adductreadily promotes incorporation of the otherwise insoluble imidazoles inthe epoxy resin component system. It is also believed that the formationof the adduct reduces the melting point of the imidazole, therebylowering the cure reaction temperature between the epoxy groups and theadduct. It is further believed that the adduct when used alone withoutother curing agents allows the curing of the epoxy resin component tooccur at significantly lower temperatures or at significantly higherrates. This permits the coating powders to be used on heat sensitivesubstrates, such as wood, without exposure of the substrate to excessiveheat that tends to deteriorate the integrity of the substrate.

[0029] The catalytic cure agent is used at between about 1 and 8 partsper hundred resin (phr), most preferably between about 1 and 4 phr.

[0030] Another curing agent that can be used in combination with the lowtemperature curing agent to enhance the curing properties isdicyandiamide. A suitable dicyandiamide curing agent is sold under thetradename Dyhard 100S by SKW Chemicals. Such dicyandiamide curing agentmay be used in the coating powder composition in an amount ranging up toabout 8 phr, preferably between about 2 and 8 phr. And, more preferablybetween about 4 and 6 phr. Due to reactivity during curing,dicyandiamide is thus not available to function as an electrostaticcharge enhancement agent when contained in epoxy resins having anothercuring agent that catalyzes the curing reaction between the epoxy resinand dicyandiamide. Thus, dicyandiamide, to function as an electrostaticcharge enhancement agent, can only be used in epoxy systems notcontaining such other curing agent. Examples of curing agents that donot permit dicyandiamide to react include phenolics and acid functionalpolyesters.

[0031] If present, the dicyandiamide curing agent is used in the coatingpowder compositions in an amount ranging up to about 8 phr, preferablybetween about 2 and 8 phr. And, more preferably between about 4 and 6phr.

[0032] If polyester resins are used, one method of achieving lowtemperature cure is using acid functional polyester and GMA containingcrosslinking agents. A typical formulation is provided in the technicalbulletin for GMA 300 from Estron Chemical Corporation. U.S. Pat. No.5,436,311 describes matte powder coatings made from acid functionalpolyesters and glycidyl methacrylate containing resins. According toU.S. Pat. No. 5,436,311, the polyester used should be a linear polyesterhaving an acid number of 20 to 50 mg KOH/g; its functionality, thus willbe equal to 2. The functionality represents the average number ofcarboxyl groups per unit of molecular weight. Preferably, thepolyester's number-average molecular weight is between 2,200 and 6,000.It is important to respect the values of these parameters to obtainmatte coatings having good mechanical and chemical properties and goodweathering resistance. In fact, if the polyester is a branched polyester(functionality greater than 2) or if the acid number of the polyester is70 mg KOH/g, the coating obtained is not matte but glossy. The glasstransition temperature (Tg) of the polyester is preferably between50.degree. and 80.degree. C., so that the polyester remains solid at anormal storage temperature (20.degree. to 50.degree. C.), therebypreventing reagglomeration of the powdered thermosetting compositionsduring handling, transport and storage. The acid constituent of thelinear carboxyl group-containing polyester is an organic dicarboxylicacid, which can be an aromatic dicarboxylic acid, such as terephthalicacid, isophthalic acid, phthalic acid, and the like, or an aliphatic orcycloaliphatic dicarboxylic acid, such as adipic acid, succinic acid,1,4-cyclohexanedicarboxylic acid, and the like, alone or in admixture.These acids can be used in the form of the free acid, the anhydride, oran ester with a lower aliphatic alcohol.

[0033] The alcoholic constituent of the linear carboxyl group-containingpolyester is an organic dihydroxy compound, which is preferably selectedfrom aliphatic diols, such as neopentyl glycol, ethylene glycol,diethylene glycol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycolhydroxypivalate, 1,4-cyclohexanedimethanol,2,2-bis(4-hydroxycyclohexyl)propane and the like, alone or in admixture.Polyesters consisting mainly of terephthalic acid, isophthalic acid andneopentyl glycol are preferably used, but polyesters in which all theconstituents are aliphatic compounds can also be used.

[0034] The carboxyl group-containing polyester is prepared byconventional methods for synthesizing polyesters by directesterification or by transesterification, in one or more steps. In thelatter case, a hydroxyl group-containing polyester is first preparedfrom one or more organic dicarboxylic acids (or their functionalderivatives) and an excess of an organic dihydroxy compound, and thehydroxyl group-containing polyester thus obtained is then esterifiedwith an organic dicarboxylic acid to produce a linear carboxylgroup-containing polyester.

[0035] The polyester synthesis is generally carried out in a reactorequipped with a stirrer, an inert gas (nitrogen, for example) inlet andoutlet, a thermocouple, an adiabatic column, a condenser, a waterseparator and a vacuum connection tube. The esterification conditionsare the classical conditions, that is to say a conventionalesterification catalyst, such as dibutyltin oxide or n-butyltintrioctanoate, can be used in an amount of 0.01 to 0.5% by weight of thereagents, and optionally an antioxidant, for example tributyl phosphite,can be added in an amount of 0.01 to 0.5% by weight of the reagents.

[0036] Polyesterification is generally carried out at a temperaturewhich is gradually increased from 130° C. to about 200° C. to 240° C.,first under normal pressure and then under reduced pressure, suchtemperature being maintained until a polyester which has the desiredhydroxyl and/or acid number is obtained. In a two-step process, thereaction mixture containing the hydroxyl group-containing polyesterobtained in the first step is allowed to cool to 200° C., the desiredamount of dicarboxylic acid is added, the temperature is brought to 225°C. and such temperature is maintained, first under normal pressure andthen under reduced pressure until a carboxyl group-containing polyesterhaving the desired acid number is obtained, which can vary between 20and 50 mg KOH/g polyester. The degree of esterification is monitored bydetermining the amount of water formed in the course of the reaction andthe properties of the polyester obtained, for example the acid number,the molecular weight or the viscosity. At the end of the synthesis, thepolyester is cast into a thick layer, allowed to cool, and then groundinto particles having an average size of from a fraction of a mm to afew mm.

[0037] The glycidyl group-containing acrylic copolymers suitable for usein the powdered thermosetting compositions of the present invention areobtained from 5 to 30% by weight of glycidyl acrylate or glycidylmethacrylate and 70 to 95% by weight of methyl methacrylate. Optionally,up to 25% by weight of the methyl methacrylate can be replaced byanother vinyl monomer.

[0038] According to U.S. Pat. No. 5,436,311, the glycidylgroup-containing acrylic copolymer should have a number-averagemolecular weight (Mn) of between about 4,000 and about 10,000, andpreferably a number-average molecular weight within the range of frommore than 5,000 to 9,000. For better control of this molecular weightand its distribution, the monomers are polymerized in the presence of afree radical polymerization initiator and a chain transfer agent. Thepolydispersity of these copolymers Mw/Mn is preferably between 1.5 and2.5 (Mn being the number-average molecular weight and Mw being theweight-average molecular weight).

[0039] It is essential that an acrylic copolymer containing glycidylgroups is used in the composition according to U.S. Pat. No. 5,436,311.In fact, it has been discovered, that this copolymer plays an essentialrole in obtaining matte coatings having good properties of appearance,adhesion to metal substrates and weathering resistance. It has beenfound, in fact, that only compositions containing acrylic copolymers ofwhich the number-average molecular weight Mn is within the range from4,000 to 10,000 give coatings having both a matte and regular appearanceand good adhesion to a metal substrate. If the molecular weight of theacrylic copolymer is less than 4,000 the coating obtained is not mattebut glossy. If the molecular weight of the acrylic copolymer is greaterthan 10,000 the coating obtained is matte but does not adheresufficiently to a metal substrate and has defects on its surface.

[0040] The monomer composition of the acrylic copolymer is also verycritical for obtaining matte coatings. If its content of glycidylacrylate or glycidyl methacrylate exceeds 30% by weight, the coatingobtained is not matte but glossy. On the other hand, it is absolutelynecessary for the acrylic polymer to contain a preponderant amount ofmethyl methacrylate besides the glycidyl acrylate or methacrylate. Infact, if more than 25% by weight of the methyl methacrylate is replacedby (an)other vinyl monomer(s), a coating which is clearly more glossy isobtained. Examples of the vinyl monomer which can be used as a comonomerto replace the methyl methacrylate in the acrylic copolymer includealkyl acrylates and methacrylates, such as methyl acrylate, ethylacrylate, butyl acrylate, 2-ethylhexyl acrylate, ethyl methacrylate,butyl methacrylate, styrene, and the like, alone or in admixture.

[0041] The acrylic copolymer containing glycidyl groups has afunctionality of preferably between 2 and 12 (the functionalityrepresents the average number of epoxy groups per unit of molecularweight). The content of epoxy groups in the acrylic polymer ispreferably between 0.3 and 2.2 milliequivalents of epoxy/g polymer. Theglass transition temperature (Tg) of the acrylic copolymer is preferablybetween 40.degree. and 70.degree. C., and its viscosity determined bythe ICI method at 200.degree. C. (see examples) is between 8,000 and40,000 mPa.s.

[0042] The glycidyl group-containing acrylic copolymer is prepared byconventional polymerization techniques, either in mass, in emulsion, orin solution in an organic solvent. The nature of the solvent is verylittle of importance, provided that it is inert and that it readilydissolves the monomers and the synthesized copolymer. Suitable solventsinclude toluene, ethyl acetate, xylene etc. The monomers arecopolymerized in the presence of a free radical polymerization initiator(benzoyl peroxide, dibutyl peroxide, azo-bis-isobutyronitrile, and thelike) in an amount representing 0.1 to 1% by weight of the monomers.

[0043] To achieve good control of the molecular weight and itsdistribution, a chain transfer agent, preferably of the mercaptan type,such as n-dodecylmercaptan, t-dodecanethiol, isooctylmercaptan, or ofthe carbon halide type, such as carbon tetrabromide,bromotrichloromethane etc., is also added in the course of the reaction.The chain transfer agent is used in an amount of from 1.5 to 4%,preferably between 2 and 3.5% by weight of the monomers used in thecopolymerization.

[0044] A cylindrical, double-walled reactor equipped with a stirrer, acondenser, an inert gas (nitrogen, for example) inlet and outlet and ametering pump feed system is generally used to prepare the glycidylgroup-containing acrylic copolymer. Polymerization is carried out underconventional conditions. Thus, when polymerization is carried out insolution, for example, an organic solvent is introduced into the reactorand heated to reflux temperature under an inert gas atmosphere(nitrogen, carbon dioxide, and the like), and a homogeneous mixture ofthe required monomers, free radical polymerization initiator and chaintransfer agent is then added to the solvent gradually over severalhours. The reaction mixture is then maintained at reflux temperature forsome hours, while stirring, and the major portion of the solvent is thendistilled off. The copolymer obtained is subsequently freed from theremainder of the solvent in vacuo. The acrylic copolymer obtained is inthe form of a solid product, which is easily ground to give a whitishpowder.

[0045] The linear carboxyl group-containing polyester and the glycidylgroup-containing acrylic copolymer described above together constitutethe basic binder for the powdered thermosetting compositions accordingto the present invention. The results show that the acrylic copolymermust contain a certain amount of glycidyl methacrylate to allowcross-linking of the coating. An example which includes the acrylicpolymer containing no glycidyl methacrylate (0%), gives a coating havingproperties which are clearly less good (poor adhesion) and an irregularappearance. In addition, it can be seen that when the amount of glycidylmethacrylate in the acrylic copolymer is too high, it is no longerpossible to obtain matte coatings. Thus, for example, a compositionincluding copolymer which contains 40% by weight of glycidylmethacrylate, provides a glossy coating (gloss of 41) and a compositioncontaining copolymer of the same composition, but with a lower molecularweight, also provides a glossy coating. This demonstrates that, in orderto obtain a matte coating the use of an acrylic copolymer containingmore than 30% by weight of glycidyl methacrylate, with respect to thetotal weight of monomers, is not advisable.

[0046] U.S. Pat. No. 5,436,311 also relates to the use of the powderedthermosetting compositions according to the invention for thepreparation of powdered varnishes and paints which produce mattecoatings, as well as to powdered varnishes and paints obtained by meansof these compositions. The ratio of the amount of linear carboxylgroup-containing polyester to the amount of glycidyl group-containingacrylic copolymer in the powdered thermosetting compositions of thepresent invention is such that there are 0.5 to 1.5, and preferably 0.8to 1.2 equivalents of carboxyl groups per equivalent of epoxy groups inthe acrylic copolymer.

[0047] In the above mentioned patent, the formulations that led tohigher gloss contained GMA resins with epoxy equivalent weights of lessthan 300 (with 40% by weight of GMA content in the copolymer). Withconstant molecular weight if the epoxy equivalent weight is reduced, thefunctionality increases and hence the reactivity. So these formulationsthat contained acid functional polyesters with acid numbers in the rangeof 20-50 and GMA crosslinkers with epoxy equivalent weight ofapproximately 300 are typical formulations our present invention usesfor low temperature cure.

[0048] U.S. Pat. No. 5,270,416 also discloses glycidyl methacrylatecontaining resins crosslinked with carboxylic acid functionalcrosslinkers and polyesters. If acrylic resins are used, GMA resins suchas PD 7690 from Anderson Development Company can be used with DDA ascuring agent in presence of catalysts that promote this reaction.Preferred crosslinkers comprise a dicarboxylic acid component such asaliphatic dicarboxylic acids having between 9 and 20 carbon atoms.Suitable aliphatic dicarboxylic acids include for instance dodecanedioic acid (dodecyl dicarboxylic acid) and sebacic acid. The preferreddicarboxylic acid is dodecane dioic acid and the preferred level ofdicarboxylic acid crosslinker is between 9% and 25%. Suitablecrosslinkers further include low molecular weight carboxylic acidfunctional polyesters which can be utilized at levels between about 10%and 30%. Useful polyesters comprise a low molecular weight linearpolyester polymer having a number average molecular weight between about250 and 1000 preferably between 250 and 500, while the Acid No. shouldbe above about 150 and preferable between 250 and 400. Suitable lowmolecular weight polyesters comprise the esterification product of aglycol with dicarboxylic acid, where linear aliphatic glycols esterifiedwith lesser equivalents of a linear saturated dicarboxylic acid havingbetween 2 and 12 linear carbon atoms such as adipic, azelaic, succinic,glutaric, pimelic, suberic or sebacic acid. Preferred and commerciallyavailable linear saturated dicarboxylic acids are adipic or azelaicacid. Minor amounts of aromatic dicarboxylic acids (anhydrides) such asphthalic, isophthalic, terephthalic, and tetrahydrophthalic can beincluded, if desired, within molecular weight range. Suitable glycolsinclude non-ether linear aliphatic glycols having 2 to 8 carbon atomssuch as 1,3 or 1,4 butylene glycol, 1,6 hexane diol, neopentyl glycol,propylene glycol and similar linear glycols or ether glycols such asdiethylene glycol and dipropylene glycol and bisphenol ethers. Thepreferred glycol is ethylene glycol. World patent publication WO99/16838 discloses achieving low gloss in these compositions usingstyrene maleic anhydride copolymers as a gloss reducing agent.

[0049] U.S. Pat. Nos. 4,147,737 and 5,168,110 disclose other glycidylfunctional crosslinkers that can be used with acid functional polyestersas thermosetting powder coating compositions. Epoxy resins such as thatare based on bisphenol A can also be used as crosslinkers to form hybridpowder coatings. Such epoxy resins are discussed above and disclosed inU.S. Pat. No. 5,721,052.

[0050] European Patent Application 0 214 448 (A2) discloses compositionscontaining acid group containing acrylic polymer and epoxy resins.

[0051] The acid group-containing acrylic polymer is preferably acarboxylic acid group-containing acrylic polymer and can be formed byreacting apolymerizable alpha, beta-ethylenically unsaturated carboxylicacid with one or more other polymerizable alpha, beta-ethylenicallyunsaturated monomers, particularly vinyl aromatic monomers and estera ofalpha, beta-ethylenically unsaturated carboxylic acids.

[0052] Examples of the carboxylic acid group-containing monomers whichcan be used are acrylic acid and methacrylic acid, which are preferred,as well as crotonic acid, itaconic acid, fumaric acid, maleic acid,citraconic acid, and the like, as well as monoalkyl esters ofunsaturated dicarboxylic acids such as itaconic acid, fumaric acid,maleic acid, citraconic acid, and the like. The acid group-containingmonomer is preferably present in the polymer in amounts of about 3 to 25percent, more preferably from about 5 to 20 percent by weight based ontotal weight of the monomers. Amounts less than 3 percent result inpoorer solvent resistance and poorer mechanical strength of the coating.

[0053] Examples of vinyl aromatic compounds are monofunctional vinylaromatic compounds such as styrene which is preferred, toluene andalkyl-substituted styrenes such as toluene and chloro-substitutedstyrene such as chlorostyrene. The vinyl aromatic monomer is preferablypresent in the composition in amounts of about 25 to 75, more preferablyfrom about 40 to 75 by weight based on total weight of the monomers.Amounts less than 25 percent result in poor detergent resistance,whereas amounts greater than 75 percent result in poor flexibility.

[0054] Examples of the esters of the alpha, beta-ethylenicallyunsaturated acids are esters of acrylic and methacrylic acid and includemethyl methacrylate, ethyl acrylate, butyl acrylate, 2-theylhexylacrylate, dodecenyl acrylate, methyl methacrylate, ethyl methacrylate,N-butyl methacrylate, and 2-ethylhexyl methacrylate. Preferably, theseesters are present in amounts of about 5 to 70, more preferably fromabout 10 to 50 percent by weight, based on total weight of the monomers.Amounts less than 5 percent result in brittle coatings, whereas amountsgreater than 70 percent result in poor detergent resistance.

[0055] In addition to the vinyl aromatic compounds and the esters ofacrylic an methacrylic acid, other ethylenically unsaturatedcopolymerizable monomers may be used. Examples include nitriles such asacrylonitrile, vinyl and vinylidene halides such as vinyl chloride andvinylidene fluoride and vinyl esters such as vinyl acetate. Theseadditional monomers are present in amounts of about 0 to 40, preferablyfrom 0 to 30 percent by weight based on total weight of monomers.

[0056] In preparing the acid group-containing acrylic polymer, thevarious monomers are mixed together and are reacted by conventional freeradical initiated polymerization processes. Among the free radicalinitiators which may be used are benzoyl peroxide, tertiarybutylhydroperoxide, ditertiarybutyl peroxide, azobis (2-methylpropionitirle),and so forth. The polymerization is preferably carried out in solutionusing a solvent in which the monomers are soluble such as toluene orxylene. At the completion of the polymerization, the reaction mixturecan be devolatilized such as by placing under vacuum to remove theorganic solvent and recovering the polymer as a sold material.Alternately, the polymer can be precipitated and subsequently dried.Usually, the devolatilized polymer will contain less than 1 percent byweight of materials that volatilize at the temperatures used for curingthe coatings.

[0057] The acid group-containing acrylic polymer can also be prepared byemulsion polymerization, suspension polymerization, bulk polymerizationor suitable combinations thereof. These techniques are well known in theart.

[0058] The acid group-containing acrylic polymer is preferably used inamounts of about 40 to 80, more preferably 50 to 70 percent by weightbased on weight of resin solids. Amounts less than 40 percent by weightare not preferred because of poor heat and color stability, whereasamounts greater than 80 percent by weight result in poor flexibility andcorrosion resistance.

[0059] Besides the carboxylic acid group-containing copolymer, thethermosetting powder coating composition of the invention preferablycontains another acid group-containing material which is either adibasic acid derived from a C₄ to C₂₀ aliphatic dicarboxylic acid or acarboxylic acid group-terminated polyester. These particular materialsare desirable because they provide flexibility and impact resistance inthe resultant coating. Among the aliphatic dicarboxylic acids which maybe used include adipic acid, subaric acid, azelaic acid, sebacic acid,and dodecanedioic acid. Preferably, the aliphatic dicarboxylic acid is asolid at room temperature.

[0060] Acrylic resins for hybrid coating powders are available from S CJohnson. Product designations include SCX820, 831, and 848.

[0061] In all of the above compositions, suitable catalysts can be usedto enhance low temperature cure characteristics. For all of thethermosetting compositions involving acid functional and glycidylfunctional materials suitable catalysts can be chosen from amines (suchas DBU), ammonium salts (such as tetra butyl ammonium bromide, benzyltrimethyl ammonium chloride), phosphine (such as triphenyl phosphine),phosphonium salts (such as ethyl triphenyl phosphonium bromide),imidazole (such as 2-methyl imidazole, 2-phenyl imidazole), imidazoleadducts (such as P101 from shell, HT 3261 from Ciba Geigy) can be used.U.S. Pat. Nos. 5,169,473 and 4,868,059 disclose catalysts useful forcrosslinking glycidyl containing resins. Examples of catalyst that arediscussed in these patents are compounds containing amine, phosphine,heterocyclic nitrogen, ammonium, phosphonium, arsonium or sulfoniummoieties. Especially preferred are the alkyl-substituted imidazoles;2,5-chloro-4-ethyl imidazole; and phenyl substituted imidazoles, andmixtures thereof. Even more preferred are 2-methyl imidazole; 2-ethyl,4-methyl imidazole; 1,2-dimethylimidazole; and 2-phenyl imidazole.Especially preferred is 2-methyl imidazole. Particularly suitablecatalysts are those quaternary phosphonium and ammonium compounds suchas, for example, ethyltriphenylphosphonium chloride,ethyltriphenylphosphonium bromide, ethyltriphenylphosphonium iodide,ethyltriphenylphosphonium acetate, ethyltriphenylphosphonium diacetate(ethyltriphenylphosphonium acetate acetic acid complex),ethyltriphenylphosphonium tetrahaloborate, tetrabutylphosphoniumchloride, tetrabutylphosphonium bromide, tetrabutylphosphonium iodide,tetrabutylphosphonium acetate, tetrabutylphosphonium diacetate(tetrabutylphosphonium acetate acetic acid complex),tetrabutylphosphonium tetrahaloborate, butyltriphenylphosphoniumtetrabromobisphenate, butyltriphenylphosphonium bisphenate,butyltriphenylphosphonium bicarbonate, benzyltrimethylammonium chloride,benzyltrimethylammonium hydroxide, benzyltrimethylammoniumtetrahaloborate, tetramethylammonium hydroxide, tetrabutylammoniumhydroxide, tetrabutylammonium tetrahaloborate, and mixtures thereof andthe like.

[0062] Other suitable catalysts include tertiary amines such as, forexample, triethylamine, tripropylamine, tributylamine,benzyldimethylamine, imidazoles such as 2-methylimidazole mixturesthereof and the like.

[0063] Other suitable catalysts include ammonium compounds such as, forexample, triethylamine.HCl complex, triethylamine.HBr complex,triethylamine.HI complex, triethylamine.tetrahaloboric acid complex,tributylamine.HCl complex, tributylamine.HBr complex, tributylamine.HIcomplex, tributylamine.tetrahaloboric acid complex,N,N′-dimethyl-1,2-diaminoethane.tetrahaloboric acid complex, andmixtures thereof and the like.

[0064] Other suitable catalysts include quaternary and tertiaryammonium, phosphonium, and arsonium adducts or complexes with suitablenon-nucleophilic acids such as, for example, fluoboric, fluoarsenic,fluoantimonic, fluophosphoric, perchloric, perbromic, periodic, mixturesthereof and the like. U.S. Pat. No. 5,169,473 discloses latent catalyststhat are useful for the present invention.

[0065] U.S. Pat. Nos. 5,304,332 and 5,480,726 disclose a process wherethe hardness and appearance of in-mold coatings are improved by theaddition of melamine or benzoguanamine to an unsaturated polyestercoating powder. Such coating powder is applied to a metal mold. Incontrast, the present invention relates to coating powder compositionscontaining inert nitrogen-containing compounds with enhanced chargingcharacteristics when applied to non-conductive substrates.

[0066] When unsaturated polyester thermosetting resins, such asdescribed in U.S. Pat. No. 5,480,726 are used in the present invention,a surface curing initiator must be included in such unsaturatedpolyester composition. Suitable initiators include a cobalt redoxcatalyst and thermal initiator, and peroxide to obtain surface curing ofthe coating powder resin. U.S. Pat. No. 6,048,949 and European publishedPatent Application No. 0148637 A2 disclose metal containing organiccompounds that provide the necessary surface cure for the presentinvention. The in-mold coating described in U.S. Pat. No. 5,480,726 willnot cure at the surface because air prevents surface curing of suchformulations. During the curing step described in U.S. Pat. No.5,480,726, curing is performed in a closed area having air inhibition toobtain a surface cure. Should radiation curing such as by UV radiationbe used, a UV initiator must be added to the composition.

[0067] It should be understood that the term in the claims “consistingessentially of” includes as optional components of the coating powdercomposition at least the ingredients shown below.

[0068] In addition to the above-described thermosetting resin polymer,the coating powder composition may optionally include as anothercomponent, a texturing agent for achieving the desired grainy texturedeffect of the finish. The texturing agents which may be employed in thethermosetting powder coating compositions of the present invention areexemplified by, without limitation, organophilic clays, such as anorganophilic clay sold under the tradename Bentone 27 and Bentone 38 byNL Chemicals, which are trialkylarylammoniumhectorite andtetraalkylammoniumsmectite, respectively, rubber particles, such asacrylonitrile butadiene copolymers, including those sold under thetradename Nipol 1422 and 1411 by Zeon Chemicals, and thermoplasticpolymers, such as polypropylene. The amount of texturing agent useddetermines the coarseness or fineness of the texture. The texturingagent is used in a range up to about 30 phr, preferably between about 1and 20 phr, and, most preferably between about 2 and 10 phr. If rubberparticles are used as the texturing agent, it is generally preferred toincorporate them in the powder coating composition in an amount rangingbetween about 5 and 30 phr. And more preferably between about 10 and 20phr. It is believed that the texturing agent contributes to the highviscosity and low melt flow of the powder coating composition leading tothe textured finish and, thus, provides for better edge coverage andhiding of surface imperfections of wood substrates.

[0069] The thermosetting powder coatings of the present invention mayalso desirably include as another component, a flow control agent. Theflow control agents which may be employed in the thermosetting powdercoating compositions are exemplified by, without limitation, acrylicresins. These acrylic resins are generally liquids which have beenconverted to powder form by absorption onto silica-type materials. Apreferred flow control agent is sold under the tradename Resiflow P-67acrylic resin by Estron Chemical, Inc., which is a 2-propenoic acid,ethyl ester polymer. Another preferred flow control agent is sold underthe tradename Benzoin by DSM, Inc., which is a2-hydroxy-1.2-diphenylethanone crystalline sold that is believed to keepthe molten coating open for a suitable time to allow outgassing to occurprior to the formation of the hard set film. The flow control agent isused in a range between about 1 and 5 phr, preferably between about 1.5and 2.5 phr.

[0070] Fumed silica and aluminum oxide may also be included as a powderdry flow additive. An example of fumed silica is sold under thetradename Cab-O-Sil by Cabot Corporation. An example of aluminum oxideis sold under the tradename Aluminum Oxide C by Degussa Corporation.

[0071] In addition, the thermosetting powder coating compositions maycontain pigments as another component. Any of the usual pigments may beemployed in the thermosetting powder coating of the invention to obtainthe desired color and opacity. Examples of useful pigments for the blacktextured powder coatings include, without limitation, carbon black andblack iron oxide. A preferred carbon black pigment is sold under thetradename Raven 22 and Raven 1255 by Columbian Chemical Company. Anexample of a useful pigment for white textured powder coatings include,without limitation, titanium dioxide. The pigment is used in a range upto about 100 phr, more preferably between about 1 and 4 phr for a blacktextured finish and between about 15 and 80 phr for a white texturedfinish.

[0072] The thermosetting powder coating compositions of this inventionmay contain extenders or fillers as another component. If a texturedfinish is desired, the extender loading can be rather high to lower themelt flow of the powder coating and allow the molten coating to curewhile retaining some of the finish of the powder particles as applied.The level of extenders can also be used to control the coarseness orfineness of the finish. The extenders which may be employed in thethermosetting powder coating compositions of the present invention areexemplified by, without limitation, calcium carbonate, barium sulfate,wollastonite, and mica. The extender is used in a range up to 120 phr,more preferably between about 10 and 80 phr.

[0073] In addition to the above components, the thermosetting powdercoating compositions of this invention may also contain the usualadditives common to powder coatings. These additives include, withoutlimitation, gloss control waxes, such as polyethylene, slip additives,such as Teflon and siloxanes, and the like.

[0074] When metallic substrates are coated with electrostaticallycharged coating powder particles, such particles are attracted to thegrounded metal part and thereby significant the coating thicknesses, onthe order of up to 20 mils can be obtained. However, electricallynon-conductive parts cannot dissipate charge and thereby cannot obtainsubstantial build-up of the powder on the surface of the part. Thisinvention solves such problem through the addition of an electrostaticcharge enhancement agent into the above discussed thermosetting resinouspolymer composition. It is believed further that the charge enhancingagents of the invention dissipate the charge after the coating isdeposited onto the non-conductive substrate and thus facilitate thedeposition of more powder.

[0075] Nitrogen-containing inert materials such as melamine, urea,dicyandramide, and benzoguanamine are preferred electrostatic chargeenhancement agents due to availability considerations but it is to beunderstood that derivatives of melamine and urea, such as ammelaine,melamammelide, ammelide, melem, melam, ureidomelamine, polyurea,oligomeric melamine, ortho toyl biguanide (OTB) and others can be usedin the invention.

[0076] The electrostatic charge enhancement compounds of the inventionmay be incorporated into the coating powders of the present invention inan effective amount to enhance the electrostatic chargeability of thecoating powder. Typically, at about at least 1 part of the compound per100 parts of resin (1 ppr) are incorporated. Such compounds may beincorporated in amounts of about 1 to about 30 parts, based upon 100parts of resin. The lower limit is selected because at least about 1part is needed to enhance charge for commercial coating activity. Theupper limit of 30 parts constitutes an amount where appreciably moreenhancement is not required. Preferably about 1 part to 20 parts byweight may be employed because the incremental increase is not very highafter 20 part, and chargeability is extremely good at 20 parts. Mostpreferably, a range of from about 5 to 20 parts may be used.

[0077] Examples of suitable electrically non-conductive, heat sensitiveheat sensitive substrates useful in the present invention include wood,such as hardwood, hard board, laminated bamboo, wood composites, such asparticle board, electrically conductive particle board, fiber board,medium density fiber board, masonite board, laminated bamboo, and othersubstrates that contain a significant amount of wood. Wood substratesare typically used in kitchen cabinetry, shelving and storage units,home and business furniture, computer furniture, etc.

[0078] Other non-conductive, heat sensitive substrates are plastics,such as ABS, PPO, SMC, polyolefins, acrylics, nylons and othercopolymers which usually will warp or outgas when coated and heated withtraditional heat curable powders. The plastics are typically used inautomotive parts. Still other heat sensitive substrates include paper,cardboard, and composites and components with a heat sensitive aspect,and the like.

[0079] In addition to coating non-conductive substrates, it is alsocontemplated that the coating powders of the invention canadvantageously be used to coat conductive substrates such as thin metalsubstrates where it is desired to cure the coating at temperatures belowabout 300° F. to prevent damage, such as warpage, to the thin substrate.Other conductive substrates such as those having complex shapes, canbeneficially use the coating powders of the invention due to enhancedchargeability. The coating powders of the present invention also canadvantageously be used to coat substrates having comers and otherdifficult-to-coat portions. Curing temperatures less than or greaterthan 300° F. can be used.

[0080] Non-conductive, heat resistant substrates include glass,ceramics, ceramic tiles, carbon, and graphite.

[0081] The coating powders of this invention are applied in dry, freeflowing, solid powder form over the substrate to be coated. Preferably,the powders are sprayed onto the substrate by well known electrostaticpowder spray techniques, such as corona discharge or triboelectricelectrostatic spray techniques.

[0082] Next, the powders are exposed to sufficient heat to melt, leveland flow out the powders into a continuous molten film having thedesired smoothness, and activate the cure. Heating may take place ineither infrared or convection ovens, or a combination of both.Sufficient outgassing of substrate volatiles simultaneously occursduring the flow out step to climinate surface defects, such as blistersand pin holes.

[0083] In addition, certain powders of the invention may be cured byradiation such as ultraviolet or electron beam or a combination ofthermal and radiation curing such as taught in U.S. Pat. No. 5,923,473.

[0084] The invention will be further clarified by a consideration of thefollowing specific examples which are intended to be purely exemplary ofthe invention.

[0085] To evaluate the charging characteristics of the various coatingpowders, a simple test was developed. A known amount of material wasmanually sprayed by a corona discharge gun and applied while maintainingall spray parameters constant over same surface area. The coverageobtained on of the part on the edges and on the back provides a measureof the charging characteristics of the powder coating. Charging wasrated 1 through 10, with 10 being the best charging characteristic.TABLE 1 Raw Material 100-26-1 100-26-1 100-26-3 100-32-1 100-32-2 Acid80 80 80 80 80 functional polyester (1) Glycidyl 20 20 20 20 20methacrylate resin (2) Irgafos 168 0.8 0.8 0.8 (3) Irganox 1010 0.8 0.80.8 (3) Powdertex 0.2 0.2 0.2 61 (4) Bentone 38 0.3 0.3 0.3 2 2 (5) HT3261 (6) 3 3 3 3 3 Pigments 0.5 0.5 0.5 2 2 Calcium 60 60 Carbonate (7)Melamine 0 5 10 2 20

[0086] Properties of the above compositions Property 100-26-1 100-26-2100-26-3 100-32-1 100-32-2 Geltime (1) 98 100 95 110 102 Gloss High HighHigh High High (visual) Charge- 0 5 8 3 10 ability rating (2) SolventSlight rub Slight rub Slight rub Slight rub Slight rub Resistance offoff off off off

[0087] The formulations used are typical formulations set forth in U.S.Pat. No. 5,436,311. Suitable catalysts were incorporated to provide thenecessary low temperature cure. The acid functional polyester may havean acid number between 20 and 50 with functionality of 2 and above. TheGMA crosslinker may have equivalent weight between 250 and 600 withfunctionality of 4 and above. The stoichiometry may vary from 0.5 to 2.0of the GMA crosslinker to the polyester resin. Thus the ratio of thepolyester to GMA may vary between 90:10 to 70:30. This ratio will alsovary based upon the equivalent weight of the GMA crosslinker used. Thelevel of catalyst (HT 3261) may vary between 0.5 phr to 10 phr. In theexamples given above the catalyst was used at 3 phr. The solventresistance indicates that these formulations are capable of curing atlow temperatures on heat sensitive substrates and provide good coatingproperties. These results also indicate that the addition of melamine tothe powder coating compositions did not change the cure speed (geltimeand solvent resistance) or the appearance (visual gloss) of the coating.The chargeability of the product was very substantially improved withthe addition of melamine powder. TABLE 2 Raw Material 100-27-1 100-27-4100-25-3 100-25-5 SCX 821 (1) 35 35 SCX 848 (1) 15 15 SCX 815 (1) 92 92GT 7072 (2) 50 50 PT 810 (3) 50 50 8 8 Resiflow P 67 (4) 1.4 1.4 1.0 1.0Benzoin (5) 0.8 0.8 0.8 0.8 Raven 1255 (6) 2 2 2 2 Calcium Carbonate 4545 45 45 Powdertex 61 2 2 2 2 2 Phenyl 2 2 imidazole Actiron 43-65 (7)0.5 0.5 Melamine 5 Dicyandiamide 10

[0088] Properties 100-27-1 100-27-4 100-25-3 100-25-5 Geltime (sec) 120111 82 90 60° Gloss 1.5 2.0 15 13 Chargeability rating 0 6 0 6 SolventResistance Slight Slight No rub No rub rub off rub off off off

[0089] Compositions 100-27-1 and 100-27-2 are acrylic epoxy hybrid, andcompositions 100-25-3 and 100-25-5 are acrylic TGIC hybrid. The acidfunctional acrylic resin may be selected from a whole range of productswith acid number ranging between 40 (SCX 815) and 220 (SCX 848). Theepoxy functional resin may be chosen from bis A type epoxies, epoxyphenol novolac, epoxy cresol novolac and TGIC. The stoichiometry of acidgroups to epoxy groups can be adjusted to be between 0.5 and 2.0.Suitable catalysts such as 2-phenyl imidazole for acrylic epoxy hybridand tetrabutyl ammonium bromide for the acrylic TGIC hybrid are used toachieve low temperature cure suitable for our application. The aboveresults also illustrate that the addition of melamine and dicyandiamidedoes not alter cure speed and appearance but functions to significantlyimprove the electrostatic chargeability of the coating powder.

I claim:
 1. A coating powder consisting essentially of an inertnitrogen-containing compound which is a member selected from the groupconsisting of melamine, urea, benzoguanamine, derivatives of melamine,derivatives of urea, and derivatives of benzoguanamine, said compoundbeing present in an effective amount to enhance the electrostaticchargeability of said coating powder, balance essentially athermosetting resin polymer.
 2. The coating powder of claim 1, whereinsaid thermosetting resin polymer is capable of curing at a temperaturebelow about 300° F.
 3. The coating powder of claim 1, wherein said inertnitrogen-containing compound is present in an amount of at least 1 partby weight per 100 parts by weight of resin.
 4. The coating powder ofclaim 3, wherein said inert nitrogen-containing compound is present inan amount from about 1 to about 30 parts by weight per 100 parts byweight of resin.
 5. The coating powder of claim 4, wherein said inertnitrogen-containing compound is present in an amount from about 1 toabout 20 parts by weight per 100 parts by weight of resin.
 6. Thecoating powder of claim 5, wherein said inert nitrogen-containingcompound is present in an amount of from about 5 to about 20 parts byweight per 100 parts by weight of resin.
 7. The coating powder of claim1, wherein said thermosetting resin polymer is a member selected fromthe group of epoxies, saturated polyesters, polyester-epoxy hybrids,unsaturated polyesters containing a surface curing initiator, acrylics,epoxy functional acrylics, and admixtures thereof.
 8. A coating powderconsisting essentially of about 5 to about 20 parts by weight, basedupon 100 parts by weight of thermosetting epoxy resin polymer ofmelamine, balance essentially a thermosetting epoxy resin capable ofcuring at a temperature below about 300° F.
 9. A process for coating anon-conductive substrate comprising: a) Providing a coating powderconsisting essentially of an inert nitrogen-containing compound which isa member selected from the group consisting of melamine, urea,benzoguanamine, derivatives of melamine, derivatives of urea, andderivatives of benzoguanamine, said compound being present in aneffective amount to enhance the electrostatic chargeability of saidcoating powder, balance essentially a thermosetting resin polymer; b)Applying an electrostatic charge to said coating powder; c) Contacting anon-conductive substrate with said electrostatically charged powder toform a powder coating on said substrate; and d) Curing said powder toform a cured coating on said substrate.
 10. The process of claim 9,wherein curing of said powder is conducted at a temperature below about300° F.
 11. The process of claim 9, wherein said substrate is a memberselected from the group consisting of wood, plastic, paper, andcardboard.
 12. The process of claim 11, wherein said inertnitrogen-containing compound is melamine and said substrate is wood. 13.The process of claim 9, wherein said curing is thermal curing.
 14. Theprocess of claim 9, wherein said curing is radiation curing.
 15. Theprocess of claim 9, wherein said curing is radiation and thermal curing.16. The process of claim 9, wherein said inert nitrogen-containingcompound is present in an amount of at least about 1 part by weight per100 parts by weight of resin.
 17. The process of claim 16, wherein saidinert nitrogen-containing compound is present in an amount of at leastabout 1 part by weight to about 30 parts by weight per 100 parts byweight of resin.
 18. The process of claim 17, wherein said inertnitrogen-containing compound is present in an amount of at least about 1part by weight to about 20 parts by weight per 100 parts by weight ofresin.
 19. The process of claim 18, wherein said inertnitrogen-containing compound is present in an amount of at least about 5parts by weight to about 20 parts by weight per 100 parts by weight ofresin.
 20. A powder coated article comprising a non-conductive substratecoated with a cured coating powder consisting essentially of an inertnitrogen-containing compound which is a member selected from the groupconsisting of melamine, urea, benzoguanamine, derivatives of melamine,derivatives of urea, and derivatives of benzoguanamine, said compoundbeing present in an effective amount to enhance the electrostaticchargeability of said coating powder, balance essentially athermosetting resin polymer.
 21. The powder coated article of claim 20,wherein said thermosetting resin polymer is capable of curing at atemperature below about 300° F.
 22. The powder coated article of claim20, wherein said inert nitrogen-containing compound is present in anamount of at least about I part by weight per 100 parts by weight ofresin.
 23. The powder coated article of claim 22, wherein said inertnitrogen-containing compound is present in an amount of at least about 1part by weight to about 30 parts by weight per 100 parts by weight ofresin.
 24. The powder coated article of claim 23, wherein said inertnitrogen-containing compound is present in an amount of at least about 1part by weight to about 20 parts by weight per 100 parts by weight ofresin.
 25. The powder coated article of claim 24, wherein said inertnitrogen-containing compound is present in an amount of at least about 5parts by weight to about 20 parts by weight per 100 parts by weight ofresin.
 26. The powder coated article of claim 20 wherein saidthermosetting resin polymer is a member selected from the group ofepoxies, saturated polyesters, polyester-epoxy hybrids, unsaturatedpolyesters containing a surface curing initiator, acrylics, epoxyfunctional acrylics, and admixtures thereof.
 27. The powder coatedarticle of claim 20, wherein said non-conductive substrate is a memberselected from the group consisting of glass, ceramics, ceramic tile,carbon, and graphite.
 28. A powder coated article comprising anon-conductive substrate coated with a cured coating powder consistingessentially of about 1 to 30 parts by weight, based upon 100 parts byweight of thermosetting epoxy resin polymer, of melamine, balanceessentially a thermosetting epoxy resin polymer capable of curing at atemperature below about 300° F.
 29. A coating powder consistingessentially of dicyandiamide in inert form, said dicyandiamide beingpresent in an effective amount to enhance the electrostaticchargeability of said coating powder, balance essentially athermosetting resin polymer which is a member selected from the groupepoxies which do not contain a curing agent which catalyzes a curingreaction between said epoxies and dicyandiamide, saturated polyesters,polyester-epoxy hybrids, unsaturated polyesters containing a surfacecuring initiator, acrylics, epoxy functional acrylics, and admixturesthereof
 30. The coating powder of claim 29, wherein said thermosettingresin polymer is capable of curing at a temperature below about 300° F.31. The coating powder of claim 29, wherein said inertnitrogen-containing compound is present in an amount from about 1 toabout 30 parts by weight per 100 parts by weight of resin.
 32. A processfor coating a non-conductive substrate, comprising a) Providing acoating powder consisting essentially of dicyandiamide in inert form,said dicyandiamide being present in an effective amount to enhance theelectrostatic chargeability of said coating powder, balance essentiallya thermosetting resin polymer which is a member selected from the groupepoxies which do not contain a curing agent which catalyzes a curingreaction between said epoxies and dicyandiamide, saturated polyesters,polyester-epoxy hybrids, unsaturated polyesters containing a surfacecuring initiator, acrylics, epoxy functional acrylics, and admixturesthereof; b) Applying an electrostatic charge to said coating powder; c)Contacting a non-conductive substrate with said electrostaticallycharged powder to form a powder coating on said substrate; and d) Curingsaid powder to form a cured coating on said substrate.
 33. The processof claim 32, wherein curing of said powder is conducted at a temperaturebelow about 300° F.
 34. The process of claim 32, wherein said substrateis a member selected from the group consisting of wood, plastic, paper,and cardboard.
 35. The process of claim 34 wherein said inertnitrogen-containing compound is melamine and said substrate is wood. 36.The process of claim 32, wherein said curing is thermal curing.
 37. Apowder coated article comprising a non-conductive substrate coated witha cured coating powder consisting essentially of dicyandiamide in inertform, said dicyandiamide being present in an effective amount to enhancethe electrostatic chargeability of said coating powder, balanceessentially a thermosetting resin polymer which is a member selectedfrom the group epoxies which do not contain a curing agent whichcatalyzes a curing reaction between said epoxies and dicyandiamide,saturated polyesters, polyester-epoxy hybrids, unsaturated polyesterscontaining a surface curing initiator, acrylics, epoxy functionalacrylics, and admixtures thereof.
 38. The powder coated article of claim37, wherein said thermosetting resin polymer is capable of curing at atemperature below about 300° F.
 39. The powder coated article of claim37 wherein said inert nitrogen-containing compound is present in anamount from about 1 to about 30 parts by weight per 100 parts by weightof resin.
 40. The powder coated article of claim 39, wherein said inertnitrogen-containing compound is present in an amount from about 1 toabout 20 parts by weight per 100 parts by weight of resin.
 41. A processfor coating a conductive substrate comprising: a) Providing a coatingpowder consisting essentially of an inert nitrogen-containing compoundwhich is a member selected from the group consisting of melamine, urea,benzoguanamine, derivatives of melamine, derivatives of urea, andderivatives of benzoguanamine, said compound being present in aneffective amount to enhance the electrostatic chargeability of saidcoating powder, balance essentially a thermosetting resin polymer; b)Applying an electrostatic charge to said coating powder; c) Contacting aconductive substrate with said electrostatically charged powder to forma powder coating on said substrate; and d) Curing said powder to form acured coating on said substrate.
 42. The process of claim 41, whereincuring of said powder is conducted at a temperature below about 300° F.43. The process of claim 41, wherein said substrte is metal.
 44. Aprocess for coating a conductive substrate comprising: a) Providing acoating powder consisting essentially of a metal, said compound beingpresent in an effective amount to enhance the electrostaticchargeability of said coating powder, balance essentially athermosetting resin polymer; b) Applying an electrostatic charge to saidcoating powder; c) Contacting a conductive substrate with saidelectrostatically charged powder to form a powder coating on saidsubstrate; and d) Curing said powder to form a cured coating on saidsubstrate.
 45. A powder coated article comprising a conductive substratecoated with a cured coating powder consisting essentially of an inertnitrogen-containing compound which is a member selected from the groupconsisting of melamine, urea, benzoguanamine, derivatives of melamine,derivatives of urea, and derivatives of benzoguanamine, said compoundbeing present in an effective amount to enhance the electrostaticchargeability of said coating powder, balance essentially athermosetting resin polymer.
 46. A process for coating a conductivesubstrate, comprising a) Providing a coating powder consistingessentially of dicyandiamide in inert form, said dicyandiamide beingpresent in an effective amount to enhance the electrostaticchargeability of said coating powder, balance essentially athermosetting resin polymer which is a member selected from the groupepoxies which do not contain a curing agent which catalyzes a curingreaction between said epoxies and dicyandiamide, saturated polyesters,polyester-epoxy hybrids, unsaturated polyesters containing a surfacecuring initiator, acrylics, epoxy functional acrylics, and admixturesthereof, b) Applying an electrostatic charge to said coating powder; c)Contacting a conductive substrate with said electrostatically chargedpowder to form a powder coating on said substrate; and d) Curing saidpowder to form a cured coating on said substrate.
 47. A powder coatedarticle comprising a conductive substrate coated with a cured coatingpowder consisting essentially of dicyandiamide in inert form, saiddicyandiamide being present in an effective amount to enhance theelectrostatic chargeability of said coating powder, balance essentiallya thermosetting resin polymer which is a member selected from the groupepoxies which do not contain a curing agent which catalyzes a curingreaction between said epoxies and dicyandiamide, saturated polyesters,polyester-epoxy hybrids, unsaturated polyesters containing a surfacecuring initiator, acrylics, epoxy functional acrylics, and admixturesthereof.